1. Field of the Invention
The present invention relates to an inkjet printhead. More particularly, the present invention relates to an inkjet printhead with a nozzle plate designed to control an ejecting direction of ink droplets ejected through a nozzle. The present invention further relates to a method of manufacturing such a nozzle plate.
2. Description of the Related Art
Generally, an inkjet printhead is a device for printing a color image on a surface of an object by ejecting droplets of ink on a desired location of the object. Such an inkjet printhead may be classified, according to an ink ejecting method, into a thermal inkjet printhead and a piezoelectric inkjet printhead.
In the thermal inkjet printhead, ink is quickly heated by a heater, formed of a heating element, when a pulse-type current is applied to the heater. As the ink is heated, the ink is boiled to generate bubbles. The bubbles expand and apply pressure to ink in a pressure chamber, thereby ejecting ink out of the pressure chamber through a nozzle in the form of droplets. However, the thermal inkjet printhead has to heat the ink to a high temperature, e.g., several hundred degrees Celsius or more, to generate bubbles, thereby resulting in high energy consumption and thermal stress therein. Also, it is hard to increase the driving frequency of the thermal inkjet printhead because the heated ink does not readily cool down.
In the piezoelectric inkjet printhead, a piezoelectric material is used. A shape transformation of the piezoelectric material generates pressure, thereby ejecting the ink out of a pressure chamber.
FIG. 1 shows a typical piezoelectric inkjet printhead. Referring to FIG. 1, a passage plate 10 is provided with an ink passage including a manifold 13, a plurality of restrictors 12 and a plurality of pressure chambers 11. A nozzle plate 20 is provided with a plurality of nozzles 22 corresponding to the plurality of pressure chambers 11. A piezoelectric actuator 40 is disposed on the passage plate 10. The manifold 13 functions to dispense the ink from an ink storage region (not shown) to the plurality of pressure chambers 11. The restrictor 12 functions as a passage through which the ink is introduced from the manifold 13 to the pressure chamber 11. The plurality of the pressure chambers 11, which store ink to be ejected, are arranged on one or both sides of the manifold 13. The plurality of pressure chambers 11 vary in their volumes as the piezoelectric actuator 40 is driven, thereby generating pressure variations to eject ink through the nozzles and suck ink from the manifold. To realize this, a portion of the passage plate 10 which defines a top wall of each pressure chamber 11 is designed to function as a vibration plate 14 that is to be deformed by the piezoelectric actuator 40.
The piezoelectric actuator 40 includes a lower electrode 41 disposed above the passage plate 10, a piezoelectric layer 42 disposed on the lower electrode 41, and an upper electrode 43 disposed on the piezoelectric layer 42. Disposed between the lower electrode 41 and the passage plate 10 is an insulating layer 31, e.g., a silicon oxide layer. The lower electrode 41 is formed all over the top surface of the insulating layer 31 to function as a common electrode. The piezoelectric layer 42 is formed on the lower electrode 41 and is located above the pressure chambers 11. The upper electrode 43 is formed on the piezoelectric layer 42 to function as a driving electrode applying voltage to the piezoelectric layer 42.
When an image is printed using the above-described typical inkjet printhead, the resolution of the image is affected by the number of nozzles per inch. Here, the number of nozzles per inch is represented by “Channel per Inch (CPI)” and the image resolution is represented by “Dot per Inch (DPI).” The improvement of the CPI in the typical inkjet printhead generally depends on improvements in materials processing technologies, actuator improvements, etc. However, the improvement of the CPI may not keep up with demands for increasingly higher resolution (DPI) images. Therefore, a variety of technologies for printing a higher DPI image using a low CPI printhead have been developed. FIGS. 2 and 3 show examples of those technologies.
According to one example, depicted in the upper portion of FIG. 2, a plurality of nozzles 51 and 52 are arranged along two or more rows. As illustrated, the nozzles 51 arranged along a first row and the nozzles 52 arranged along a second row may be staggered. Using this array of nozzles 51 and 52, the droplets ejected from the nozzles 51 and 52 print an image, while forming a single line, as depicted in the lower portion of FIG. 2. That is, dots 61 formed by the nozzles 51, which are arranged along the first row, and the dots 62 formed by the nozzles 52, which are arranged along the second row, alternate on a print medium 60. Therefore, the image DPI formed on the print medium 60 is two times the CPI of the printhead 50.
However, in order to precisely print the image, the nozzles 51 and 52 must be arranged accurately along the respective rows. Therefore, there is a need for an arrangement system that can precisely arrange the nozzles 51 and 52. This increases the size and cost of the printhead.
According to another example, depicted in FIG. 3, the printing is performed by a printhead 70 having a low CPI and inclined at a predetermined angle Θ with respect to a print medium 80. That is, the printhead 70 is not perpendicular to a direction of travel of the print medium 80, but rather is rotated from the perpendicular by the angle Θ. As a result, intervals between dots 81 formed on the print medium 80 are less than intervals between the nozzles 71 along the printhead 70. Thus, the image DPI on the print medium 80 is higher than the CPI of the printhead 70. In this case, the greater the inclined angle Θ, the higher the DPI. However, inclining the printhead 70 foreshortens the effective coverage of the printhead 70 on the print medium 80. That is, the printing area is reduced such that, in order to obtain a printing area equal to that obtained by an uninclined printhead, the length of the printhead 70 must be increased.